1. Field of the Invention
The present invention relates to a communication terminal connected to a telephone line network and, more specifically, to a communication terminal adapted to direct dial-in service.
2. Description of the Background Art
In offices and companies, Direct Dial-In (DDI) service is used by installing a PBX (Private Branch eXchange). DDI service refers to a service in which number information necessary for extension exchange is transmitted from an external communication device to a private branch exchange of a company through a telephone line network, to enable direct connection to the extension. This service enables allocation of a unique number to each telephone connected to the private branch exchange, and a telephone having a desired extension number can directly be called, not through a station line switch board.
A facsimile machine having the DDI connection function described above, which transmits a call tone to a telephone upon detection of a DDI number registered beforehand during an incoming call accepting procedure, has been practically used. The facsimile machine is adapted such that a plurality of telephones are connected to the facsimile machine and different telephone numbers are allocated to the facsimile machine itself and to the plurality of telephones.
FIG. 4 shows an operation sequence of DDI connection in accordance with push-button method between a subscriber line exchange and a DDI-supporting terminal, connected by a pair of station lines L1 and L2. Reference numbers (S1) to (S7) in FIG. 4 are reference numbers allocated to respective operations.
Referring to FIG. 4, when a DDI subscriber is to be called, the subscriber line exchange first reverses polarities of station lines L1 and L2. Within 0.1 second after the polarity reversal, the subscriber line exchange transmits a call signal to the DDI-supporting terminal. Depending on whether a primary acknowledgement signal (formation of a DC loop) is received from the DDI-supporting terminal within 6 seconds from the polarity reversal, the subscriber line exchange operates in different manners. If the primary acknowledgement signal is received within this period, about 0.6 seconds after the reception, the subscriber line exchange transmits an extension designation signal (a number of 1 to 4 digits set in advance by a push-button signal) to the DDI-supporting terminal. If the primary acknowledgement signal does not arrive within 6 seconds, the subscriber line exchange starts an operation of disconnecting from the DDI-supporting terminal.
Depending on whether or not an extension designation reception complete signal (cut-off of DC loop) is received from the DDI-supporting terminal in a period of about 0.3 to 2 seconds after the transmission of extension designation signal is complete, the subscriber line exchange operates in different manners. If the signal does not arrive within this period, the subscriber line exchange starts an operation of disconnecting from the DDI-supporting terminal. If the extension designation reception complete signal is received, after about 0.3 seconds from the reception, the subscriber line exchange enters a standby state, waiting for a secondary acknowledgement signal from the DDI-supporting terminal.
Receiving the secondary acknowledgement signal (formation of DC loop), the subscriber line exchange forms a communication path, by returning the polarities of station lines L1 and L2 to the original polarities.
As described above, conventionally, it has been necessary to measure prescribed time periods after occurrence of some events. Therefore, not only the subscriber line exchange but also the DDI-supporting terminal must have a timer. This results in a complicated structure of the DDI-supporting terminal.
Japanese Patent Laying-Open No. 8-204942 (hereinafter referred to as “'942 application”) discloses, in facsimile equipment, a technique of preventing meaningless call operations. Specifically, in the facsimile equipment, if there is an on-hook on the caller side while a connection from the caller side is determined to be a telephone call and the user is being called (while the DC loop is cut off and subscriber line exchange is in the standby state waiting for the secondary acknowledgment signal), the on-hook operation is detected. In response to the detected on-hook operation, the facsimile equipment stops the calling operation.
Recently, as one of pre-communication notice services, a modem DDI service has been provided. According to this service, it is possible for the subscriber line exchange to detect the DDI number before receiving a call signal. By the time the call signal is received, the DDI number has already been known to the subscriber exchange and, therefore, it is possible to call a designated telephone or the like directly.
Japanese Patent Laying-Open No. 11-308363 (hereinafter referred to as “'363 application”) discloses a technique allowing execution of a desired process without charging the call fee on the call source. Specifically, utilizing the modem DDI service, the subscriber line exchange executes a process in accordance with the DDI number before line connection. As a result, the desired process can be executed without causing any call charge on the calling source.
Referring to FIG. 4, in the operation sequence of the conventional DDI service, when polarities of station lines L1 and L2 are reversed by the subscriber line exchange (S1), the DDI-supporting terminal receives the subsequently transmitted call signal (S2). In response, the DDI-supporting terminal issues the primary acknowledgement signal (forms a DC loop) (S3) and, after a prescribed time period (ta), it detects polarities of station lines and establishes the polarities. Immediately after the formation of DC loop, however, the line voltages of station lines L1 and L2 are unstable. Therefore, conventionally, sometimes the DDI-supporting terminal establishes station lines L1 and L2 in polarities different from the specification. Since the polarities of station lines are not correctly established, there has been a problem that a device on one side of communication erroneously recognizes that the other side of communication has cut off connection during communication after the second acknowledgement and erroneously disconnects the station line of itself.
The techniques disclosed in '942 and '363 applications are on the premise that the operation sequence of DDI connection is correctly performed. Therefore, the above-described problem cannot be solved by these techniques.